X11/Xlib: Window always on top

user interfacewindow-managersx11

A window should stay on top of all other windows. Is this somehow possible with plain x11/xlib? Googling for "Always on top" and "x11" / "xlib" didn't return anything useful.

I'd avoid toolkits like GTK+, if somehow possible.

I'm using Ubuntu with gnome desktop. In the window menu, there's an option "Always On Top". Is this provided by the X server or the window manager? If the second is the case, is there a general function that can be called for nearly any wm? Or how to do this in an "X11-generic" way?

Edit: I implemented fizzer's answer, now having following code:

XSelectInput(this->display, this->window,
    ButtonPressMask |
    StructureNotifyMask |
    ExposureMask |
    KeyPressMask |
    PropertyChangeMask |
    VisibilityChangeMask ); 
// ...
// In a loop:
if (XPending(this->display) >= 0)
{
    XNextEvent(this->display, &ev);
    switch(ev.type) {
    // ...
    case VisibilityNotify:
        XRaiseWindow(this->display, this->window);
        XFlush(this->display);
    break;
    // ...
    }
}

But the eventhandling and raising nearly never gets executed even my mask is correct?!

Best Answer

#define _NET_WM_STATE_REMOVE        0    // remove/unset property
#define _NET_WM_STATE_ADD           1    // add/set property
#define _NET_WM_STATE_TOGGLE        2    // toggle property

Bool MakeAlwaysOnTop(Display* display, Window root, Window mywin)
{
    Atom wmStateAbove = XInternAtom( display, "_NET_WM_STATE_ABOVE", 1 );
    if( wmStateAbove != None ) {
        printf( "_NET_WM_STATE_ABOVE has atom of %ld\n", (long)wmStateAbove );
    } else {
        printf( "ERROR: cannot find atom for _NET_WM_STATE_ABOVE !\n" );
        return False;
    }
    
    Atom wmNetWmState = XInternAtom( display, "_NET_WM_STATE", 1 );
    if( wmNetWmState != None ) {
        printf( "_NET_WM_STATE has atom of %ld\n", (long)wmNetWmState );
    } else {
        printf( "ERROR: cannot find atom for _NET_WM_STATE !\n" );
        return False;
    }

    // set window always on top hint
    if( wmStateAbove != None )
    {
        XClientMessageEvent xclient;
        memset( &xclient, 0, sizeof (xclient) );
        //
        //window  = the respective client window
        //message_type = _NET_WM_STATE
        //format = 32
        //data.l[0] = the action, as listed below
        //data.l[1] = first property to alter
        //data.l[2] = second property to alter
        //data.l[3] = source indication (0-unk,1-normal app,2-pager)
        //other data.l[] elements = 0
        //
        xclient.type = ClientMessage;
        xclient.window = mywin;              // GDK_WINDOW_XID(window);
        xclient.message_type = wmNetWmState; //gdk_x11_get_xatom_by_name_for_display( display, "_NET_WM_STATE" );
        xclient.format = 32;
        xclient.data.l[0] = _NET_WM_STATE_ADD; // add ? _NET_WM_STATE_ADD : _NET_WM_STATE_REMOVE;
        xclient.data.l[1] = wmStateAbove;      //gdk_x11_atom_to_xatom_for_display (display, state1);
        xclient.data.l[2] = 0;                 //gdk_x11_atom_to_xatom_for_display (display, state2);
        xclient.data.l[3] = 0;
        xclient.data.l[4] = 0;
        //gdk_wmspec_change_state( FALSE, window,
        //  gdk_atom_intern_static_string ("_NET_WM_STATE_BELOW"),
        //  GDK_NONE );
        XSendEvent( display,
          //mywin - wrong, not app window, send to root window!
          root, // <-- DefaultRootWindow( display )
          False,
          SubstructureRedirectMask | SubstructureNotifyMask,
          (XEvent *)&xclient );

        XFlush(display);

        return True;
    }

    return False;
}
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